libnspr: fix static build

Disable shared library in a static build.

Fixes:
http://autobuild.buildroot.net/results/6ff/6ffae151ec3eaca175923ab860cf42167871164c/
http://autobuild.buildroot.net/results/cab/cab19721c43b3256dd51ae511016bc9ea440512a/

Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Tested-by: Vicente Olivert Riera <Vincent.Riera@imgtec.com>
Tested-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

package/libnspr/libnspr.mk

@@ -21,6 +21,12 @@ LIBNSPR_CONF_OPTS = --host=$(GNU_HOST_NAME)
 LIBNSPR_CONF_OPTS += --$(if $(BR2_ARCH_IS_64),en,dis)able-64bit
 LIBNSPR_CONF_OPTS += --$(if $(BR2_INET_IPV6),en,dis)able-ipv6
 
+ifeq ($(BR2_PREFER_STATIC_LIB),y)
+LIBNSPR_MAKE_OPTS = SHARED_LIBRARY=
+LIBNSPR_INSTALL_TARGET_OPTS = DESTDIR=$(TARGET_DIR) SHARED_LIBRARY= install
+LIBNSPR_INSTALL_STAGING_OPTS = DESTDIR=$(STAGING_DIR) SHARED_LIBRARY= install
+endif
+
 ifeq ($(BR2_arm),y)
 ifeq ($(BR2_ARM_CPU_HAS_THUMB2),y)
 LIBNSPR_CONF_OPTS += --enable-thumb2